Against the backdrop of stringent European pesticide legislation, scientists at Kent-based East Malling Research have tested alternative ways of tackling a tiny mite that’s caused tremendous damage to the UK’s soft fruit supply chain
It’s difficult to fathom how a miniscule mite – the strawberry tarsonemid mite (Phytonemus (Tarsonemus) pallidus ssp. Fragariae) is between just 0.1mm and 0.3mm in size – can cause such catastrophic damage to strawberry plants – but it does. Populations of the pest have already caused tens of thousands of pounds worth of damage to UK-grown, protected strawberry crops, so scientists at East Malling Research (EMR) have carried out a project (HDC SF 133) funded by the Horticultural Development Company (HDC) that sought to find different treatments for the “microscopic menace”.
Entomologist Dr Chantelle Jay, who led the project with colleague Dr Michelle Fountain, tells Produce Business UK there has been a significant increase in the frequency and severity of attacks in UK strawberry production in the last few years, mostly due to the withdrawal of pesticides.
She says: “Strawberry tarsonemid mite can cause devastating crop losses in highly valuable protected strawberry crops – losses exceeding £10,000 per hectare per year in some instances. The problem was particularly severe in 2010 and 2011, and continues to be a problem in some crops.”
Dr Jay explains that the damage caused by the mite is most severe in everbearing strawberry varieties and in two-year plantings. She adds that those crop protection products that are permitted to be sprayed onto the crops to combat the pest often don’t even get inside the young, folded leaves of the plant where the mites like to live.
“The mite feeds mainly on the young, folded leaves of strawberry plants, making leaf surfaces rough and crinkled as they expand,” Dr Jay points out. “Sometimes the leaves turn brown and die and the whole plant usually becomes stunted. The mites also feed in the flowers and fruits – seriously affecting yield and quality.”
The three-year HDC project, which began in April 2012, therefore looked to find alternative predatory mites, that prey on strawberry tarsonemid to prevent and control the pest.
EMR’s team evaluated six predatory mite species for their effectiveness at low and high temperatures. This included the Neoseiulus cucumeris mite (still commonly referred to by its previous name, Amblyseius cucumeris), which is already a commonly used for biocontrol of the mite.
Initial experiments were carried out on strawberries grown in pots, while later experiments evaluated strawberries in grow bags. The project’s initial experiments were carried out in glasshouse and polytunnel surroundings, depending on the release requirement for each predator. In these initial trials, the strawberry plots were caged to prevent the mites from wandering between plots.
The final experiments were carried out in an environment similar to that of a commercial setting where researchers used un-caged, raised grow bags in a polytunnel together with a standard irrigation system.
The results of the trials were promising. They found one of the mites tested – Neoseiulus californicus – may also provide an effective treatment for strawberry tarsonemid mite in glasshouse crops. Meanwhile Neoseiulus cucumeris was proven to be an effective treatment in polytunnel crops, while the mite Amblyseius (Neoseiulus) barkeri also did well in the trials.
Where to next?
EMR’s findings will enable soft-fruit growers to have a wider range of options to use in integrated pest management (IPM) programmes and, fingers crossed, reduce the amount of costly damage caused by the mite.
The A. barkeri mite is already approved for use in the UK, although it is not widely marketed in this country. N. californicus is approved for use in glasshouses only.
Dr Jay recommends growers apply predatory mites early in the season before tarsonemid mite populations have the chance to build up. Many companies supply the products in a loose form in a sprinkler tube, or in slow release sachets.
She adds that predatory mites such as N. cucumeris are particularly effective as they could also be used to control western flower thrips – which are also causing widespread damage in strawberry crops.